Most of the AC's body panelling appears to be made from the aircraft alloy, Duralumin. There was probably a surplus of this material after the war. Most of these AC's had a radiator cowl made from brass (except the very early cars). Many of the inner panels are of mild steel. It is important to understand a bit about Duralumin alloy before working on it.
Duralumin is an age hardening alloy. That means that after heat treatment, it takes a period of time before its strength and hardness develops. After a bit of work hardening (e.g. after bending or rolling into shape), its tensile strength is about the same as mild steel, but from a third of the weight. However, its resistance to corrosion is poorer than for pure aluminium.
Under the American identification system, it is known as alloy 2017. It was superceded by alloy 2024 which is still available from aircraft parts suppliers. The main alloying ingredient in these alloys is copper. They are reputed to be very hard to weld, but the AC's bodywork was butt-welded together from a large number of sections.
A lot of articles seem to exaggerate the risk of cracking when folding and bending sheet material, but it is certainly advisable to anneal it when making tight bends.
There has been a lot written and talked about repairing cracks in aluminium and alloy panelling, but one has to apply the right technique to each situation. I believe there are 3 categories of crack found on the AC:
1) Cracks arising after the wood frame deteriorates, which then transfers any body stresses to the panels. The repaired cracks are subjected to little or no stress once the wood frame is repaired.
2) Cracks at each end of the two strips of panelling at the rear end of the car. These normally arise when the wood frame rots, but may also occur if the glue joints in the wood have failed. Any body twist thus causes the wood joints to flex more, and concentrates some stress on the thin panel strips. There are welds at each end, but the cracks don't always happen at those points. Even with the wood frame fully repaired, there might still be a little bit of stress on the panelling when the car twists.
3) Cracks at the bottom rear corners of the door openings. This is a design fault, with a sharp internal corner that alters in its angle when the car twists. Thus, a repaired crack is likely to reappear. On my AC, I reinforced the panelling below the crack, and glued in the crack itself, but working on the assumption that the crack will always be there, at the top of this panel. The crack should accomodate the flexing that takes place in normal service.
A common solution to a panel crack that is discussed, involves drilling a 6mm (or 1/4") hole at the end of the crack to prevent it from spreading. On the AC's body, the cracks are often invisible (and the panels non-structural), so a large hole just makes things much worse! Another solution I heard from AC owners some years ago, was to cut out the sections around a cracked weld, and weld in a new section. But I reasoned that this doubles the number of weak-points, which includes the actual welds, and the surrounding areas known as the "heat affected zone".
I've noticed an AC with sill panels separate from the main bodyshell, which is one way to eliminate the problem, albeit by adding a visible join.
The ideal solution is to find an experienced aircraft welder. Since most of the cracked areas will no longer be under stress, I opted for a suitable epoxy adhesive, with reinforcements of either fibre-glass mesh, or an additional layer of aluminium sheet. Unfortunately, one of my crack repairs did not survive the ordeal of lifting the bodyshell back onto the wood frame. This was a crack on the rear/bottom strip below the spare-wheel door. This strip is subjected to some flexing during the body lift. If a good weld repair isn't done, then the repair will need some kind of temporary reinforcement until the shell is safely on the wood frame.
Another approach might be brazing. There are various brazing kits on the market, but most are unsuitable for Duralumin and similar alloys. I tried out the American HTS2000 brazing kit. I found it easy to use on sheets of pure aluminium, but had problems trying to repair the AC's panels. The makers and UK wholesalers were helpful, but I gave up, as I was creating more damage than repairs. I have read one report of someone using it successfully on aircraft alloy 2024, so maybe it is possible to repair Duralumin with a greater level of skill?
Alan Alderwick's book recommended silicone sealant under the edges of panels when fitting to the wood frame, to keep out moisture and to prevent panel squeek. This is a good idea on small panels, but quite impractical to do on the large main bodyshell of the AC, especially with many adjustments to make, to its fit. I opted to use Waxoyl between the wood frame and the main bodyshell, as both a lubricant and a sealant.
When using silicone sealant for the small panels around the door openings, one has to work fast to hammer the edges down. Otherwise the sealant will set too thickly, and reduce the door clearances. I applied Waxoyl around the screw holes for the door-step plates, as I feared silicone sealant might raise those plates (and they are vulnerable to damage if the doors are sagging).
Environmental regulations seem to have had some impact on the effectiveness of paint strippers, but fortunately, the cellulose on the AC softens easily. I used a brand callled "Strypit". The paint becomes soft and can be mostly scraped off. It takes some time and effort to remove it all, and I used kitchen-paper with the stripper, to remove the old primer paint. Be sure to wear suitable gloves and eye-protection while using paint stripper.
It is quite viable to do small repairs to the alloy panels using an epoxy adhesive, if you are not able to weld or braze repairs. Surface prepration is very important. I used a solvent cleaner, followed by abrasive cleaning with coarse aluminium-oxide paper. This removes the oxide layer, as well as giving a rough surface as a key for the adhesive. The oxide layer starts to reform immediately, so one then has to work fast. Another clean with solvent, and then with dionised water, and don't touch the surface with fingers. Then glue the joint. I used "Araldite Metal" which sets fairly quickly (5 minute working time).
If you are repairing a crack, then it is best to open out one side with a triangular file to make a v-groove. Also, reinforce the repair behind it, depending on how much room there will be between panel and wood frame. I used fibre-glass mesh with a 0.3mm thickness. If space permitted, then an extra layer of aluminium sheet - maybe to reinforce beyond the end of the crack. As mentioned above, I don't agree with the practice of drilling a large hole at the end of the crack in this particular application.
I also took the opportunity to patch up the radio aerial hole in the roof, that had been drilled straight through the wood frame. Again, epoxy and mesh backing were employed, to hold a small disc of aluminium in the hole. I used a heavier gauge of sheet, so that it could be filed flush.
It might also be worth gluing temporary reinforcements to the outside of crack repairs, to be filed off after the bodyshell has survived its lift onto the wood frame.
Old textbooks on aircraft panel-work, have conflicting information about annealing. Some say that after softening by annealing, the metal will gradually harden with time, since it is an age hardening alloy. Others say that it will stay soft until work-hardened by rolling and folding, etc. I therefore worked on the asumption that it will age harden, and did any annealing soon before any bending.
It is debatable whether or not annealing is required for any panel-beating. But if you have to do any tight folds, annealing is essential, especially if it has already been folded and unfolded. Otherwise the surface is likely to crack.
Duralumin has to reach a temperature of about 350 to 380 degrees C (660 to 715 degrees F) to soften it. Professional body repairers use an oxy-acetylene torch, and start with a yellow flame (oxygen turned off) to coat the metal surface with black soot. Then they turn on the oxygen and heat the metal until the soot disappears, which indicates that the right temperature was achieved. I used a more modest butane torch, and a bar of bathroom soap! Coat the metal with soap (just plain soap - nothing fancy) on the side of the metal that you are going to heat. Then heat with a blow-torch until the soap turns black (not just brown). I held a thick aluminium sheet behind the panel that I was heating, to reflect the heat back, thus allowing the job to be done with a less powerful heat source. It also helps to reduce fire risk by shielding anything behind the panel. The burnt soap can be cleaned off with water.
If you have to do annealing along a fold-line, be aware that it will be soft for some small distance either side of the required fold. Care will have to be taken not to distort the panel either side of the fold. For straightening bad creases in the bodyshell, I clamped wood to the panelling either side of the crease, so I could straighten it exactly along the right line.
I got some practice by working on the detachable panels that fix to the back of the front wings. These encase a small sheet of plywood, and all the edges are folded over the wood. The bottom edge of the panel has to be completed unfolded flat, to allow a new sheet of wood to slide in. My right-hand panel was a poor fit against the door-step, so I altered the original fold-line. These panels are secured with a pair of Whitworth coachbolts (1/4") and wing-nuts. As always, I applied Waxoyl to the bolts and to seal in the edges of the panels.
With the panelling removed from the wood frame, it is an ideal opportunity to straighten out any dents or creases. The professionals use special light-weight hammers and a range of steel dollies. But one can manage with ordinary hammers and makeshift 'dollies'. The dolley is held behind the panel to absorb the impact of the hammer while helping to form the panel into the required shape. Hence a range of solid scraps of steel or hardwood to match the various curves one is dealing with. Steel is the preferred material, but it is easier to make a few wooden dollies of varying curvatures.
When hammering the panels, it is usually best to use a stroking action to avoid making lots of tiny indentations. One's finger-tips rubbed over the panel surface, is the most effective way to detect slight unevenness. The professionals go over the surface with a file, making the high points shiny, but I find the notion of filing my thin panels rather alarming!
Duralumin has a lower resistance to corrosion than pure aluminium and most of its other alloys. Therefore it is a good idea to paint any surfaces that are exposed to road spray. This includes the inside of the sill panels that are part of the main bodyshell. Also the small inner panels either side of the bulkhead and those that cover the little plywood sections that the front wings bolt to.
Aluminium requires careful preparation for painting. The special chemicals that the aero-industry employ, are somewhat hazardous and expensive. A more practical approach is cleaning with a solvent, abrading with aluminium-oxide paper, cleaning with more solvent, then dionised water. When dry, spray with an aerosol of etch-primer, recoating after about 15 minutes. A more conventional primer can be applied the following day, with undercoats and gloss top-coats in the usual manner. I used aluminium-based primer which I normally apply to wood, followed by one-part polyurethane. The glossy finish will be less prone to collect dirt and moisture than underseal.
Refitting small panels
For the panels that cover the small plywood parts, I applied Waxoyl as a sealant, since these panels have to slide onto the plywood. For the other small panels (including the ones around the door openings), I applied silicone sealant (neutral curing type, so that there's no acid released). One only has a few minutes before the sealant starts to become firmer, which may affect clearances if the panels sit too high. Holes for panel pins should be sorted out first. Pilot-holes need to be drilled into new wood. If old holes are a slack fit for the panel-pins, one can always use larger ones cut to length. The 20mm stainless steel panel-pins I ordered were a heavier gauge than the conventional mild-steel version. I could not find stainless ones shorter than 20mm, so I dipped mild steel pins in Waxoyl for rust protection.
Make sure that panel-pins are inclined, if they are fitted close to the edge of the wood frame, to avoid splitting the wood.
For the panels around the door openings, you have to work out which pin holes are for the main bodyshell that overlaps those panels. There are small panels that fit under the cantrails (above the door drop-window). I was surprised that these were separate, rather than being part of the main bodyshell. Perhaps earliers cars were like that? Maybe problems such as cracking occurred? Anyway, I fitted these before the bodyshell went on, but then realised it is better to fit them afterwards (as they might obstruct the bodyshell).
Refitting main bodyshell
This is the scariest part of the restoration! If you have painted the wood frame, then it will need protection against scratching as the bodyshell is placed back on to it. I wrapped heavy-duty polythene sheets around all the vulnerable parts of the wood frame, after applying Waxoyl to the wood. The Waxoyl makes it easy to pull out the sheets later, as well as acting as a sealant and lubricant for the panelling. Waxoyl will need to be applied around all the areas of the wood frame that may be in contact with the bodyshell.
With hindsight, I should have added some kind of reinforcements to the two thin strips of panel at the rear of the car, so that they don't flex during the bodyshell lift (and potentially break any crack repairs).
Those edges of panel to be folded, will need to be annealed, as described above.
I originally lifted the bodyshell off the car with the help of one other person. However, lifting it back on is much easier with 4 people. It is not all that heavy, but it twists and flexes very easily. While carrying the bodyshell, one has to be especially careful that the sill panels do not catch on anything. As the shell is lowered onto the wood frame, be careful that the rear end is not sitting on top of the rear wooden cross-member. The rear windows may take some persuading to go into place. If the car is going to be moved in its current state, then be sure to tie the sill panels onto the wood frame, so that they are less likely to get damaged.
The bodyshell might tend to sink a tiny fraction rearwards and downwards at the rear end, until the rear window surrounds have been hammered back over the woodwork. The front end won't sit right until the positioning is perfect. The old panel-pin holes are a useful guide, and more so with the nail holes for the guttering. If you push nails in, it will help persuade the whole thing to shift into alignment.
I had altered the rebate on one of the wooden screen pillars for a better door fit, and this meant changing the fold-line on the panelling. With the panelling folded over the screen pillars, the doors can be test fitted as well as the front wings. One needs to get a perfect alignment between the sill panelling and the wings. The doors' alignment will change slightly when their trim and new rubber stops are fitted, so keep that in mind. At this stage, I found that my new woodwork below the door-steps, was projecting too far outwards (I erred on the side of too large versus too small). So I removed some wood from the sill rails for a good panel fit, and repainted the sill rails, before pinning the bodyshell on.
The panel-pins around the door opening, need to be flush and neat, and so a careful choice of gauge and head-size has to be selected. When hammering panel-pins in, to avoid panel damage, use used a small block of steel as a punch. As mentioned above, incline the pins to avoid splitting wood. When hammering the edges over around the door opening rebates, I used a scrap of plywood and hammered this against the panelling, thus avoiding panel damage. For the screen pillars, I clamped the panelling to the wood frame to be sure it bent in the correct places. When folding over any of the edges, it is advisable to clamp (or hold) a block of wood against the adjacent area of panel, so that it folds correctly without causing unwanted creases. I used some curved scraps of plywood to help hammer over the curved window surrounds.
With the panel edges annealed, it is fairly easy to hammer them down against the wood frame. Test fit the rear and side windows, as it is possible that the wood frame is a slack fit around one or more of them, and the panel fit will need to be packed out to compensate for this.
Note that the screen pillar is not meant to be flush with the door edge, except along the decorative side mouldings. The door is recessed by about 3mm (1/8").
The boot-lid and spare-wheel door can be test fitted and the panelling pinned on so that it is flush. At the front end, the small bolts for the front wings, pass through the main bodyshell and small aluminium panels into the wood frame, so all these holes need to line up.
With wings, doors and windows successfully test fitted, the gutters can be added as the finishing touch. These were originally nailed on with about 40 nails each. I fitted them back on with small (no.4 x 3/4") stainless steel woodscrews. The gutters take a bit of work to straighten out the damage from when they were removed. There is a rather curious seal fitted under the gutter and over the small cantrail panel (i.e. just above the drop-window), which extends a few millimetres below the gutter (thus hiding the panel joins). This seal is made from a very thin calico fabric, folded double, and then coated with bitumen. The exposed part was painted over in bodywork colour. The photo below shows me test fitting this seal prior to the body panelling going back on.
With the outer bodywork all back together, you can breath a sigh of relief. Now you can get on with those easy tasks that other restorers make a big deal about :)